It is well known to employ permeable membranes to separate or selectively enrich a gas mixture. For example, membranes are used in separation of H.sub.2 from supercritical gases such as N.sub.2, CO and CH.sub.4 ; the separation of CO.sub.2 and water vapor from natural the enrichment of air by nitrogen or oxygen. Hydrogen is recovered from ammonia production plants using large scale membrane technology and likewise, the recovery of H.sub.2 from coal for the production of synthetic fuel is possible.
The fundamentals of gas separation are based upon the permeability equation, which at low pressures in the absence of strong interactions between components may be expressed as: EQU .alpha..sub.AB =P.sub.A /P.sub.B
Where:
.alpha..sub.s is referred to as the ideal separation factor. PA1 P.sub.A is the permeability of component A in the membrane. PA1 P.sub.s is the permeability of component B in the membrane. PA1 R is a tetravalent aromatic radical selected from the group consisting of: ##STR2## Z is a divalent radical selected from: ##STR3## Ar is: ##STR4## X.sub.1. . . X.sub.8 ; being defined hereinbelow: ##STR5## or mixtures thereof; Ai is: ##STR6## PA1 X.sub.1 . . . X.sub.8 are independently H, alkyl groups with 1-6 carbon atoms, preferably methyl groups; Br, Cl; or aromatic groups with 6-13 carbons. PA1 1,1-bis[4-(4-aminophenoxy)phenyl -1-pehnyl-2,2,2-triflurorethane;
The separation of gas components by polymer membranes is thought to depend on chemical affinities, kinetic diameters and structural characteristics; it being known very generally that rubbery polymers are characterized by high diffusion and relatively low selectivity while glassy polymers are characterized by lower diffusion and higher selectivities. In any given situation, however, it is impossible to predict separation in the absence of experimental data with any reasonable degree of confidence.
Much of the work in the field has been directed to developing membranes which optimize the separation factor and total flux of a given system. It is disclosed in U.S. Pat. No. 4,717,394 to Hayes that aromatic polyimides containing the residue of alkylated aromatic diamines are useful in separating a variety of gases. Moreover, it has been reported in the literature that other polyimides, polycarbonates, polyurethanes, polysulfones and polyphenyleneoxides are useful for like purposes.
It has now been found, in accordance with the present invention, that certain polyimides with the residuum of a diaryl fluorine-containing diamine moiety are useful in separation processes involving, for example H.sub.2, N.sub.2, CH.sub.4, CO, CO.sub.2, He and O.sub.2 by virtue of the fact that these polymers exhibit both high flux rate and separation factors.